Method and apparatus for providing internal table extensibility with external interface

ABSTRACT

A configurable lookup table extension system including a plurality of lookup tables arranged in an internal memory, an external memory, and a flexible controller configured to couple at least one of the plurality of lookup tables to the external memory through a single memory interface is disclosed. Implementations of this system can support the flexible allocation of IP and MAC table entries so that a router/switch can flexibly support applications suited to a particular allocation. This approach provides an efficient scheme for extending multiple internal tables to external memory via a single external interface. Further, such extensibility is also programable to allow the size and number of external tables to be configured by software. This solution can provide the flexibility of customizing table sizes for different markets and/or customer requirements.

FIELD

The invention relates generally to the field of networking, and moreparticularly to lookup table allocation.

BACKGROUND

In networking systems, routers and/or switches typically use lookuptables to facilitate or increase the speed of the forwarding of packetsof information. Such tables can be implemented in hardware, for example,as content-addressable memory (CAM), which can include static randomaccess memory (SRAM) type memory cells. Among the types of forwardingthat can be accommodated in typical systems are layer 2 (L2) and/orlayer 3 (L3) forwarding. L2 tables can include Media Access Control(MAC) and L3 tables can include Internet Protocol (IP). Other exampletable types include IP MultiCast (MC), IP Next Hop Table (NHT), and IPLongest Prefix Match (LPM) tables. Internal (i.e., on-chip) tables canbe used for the L2/L3 forwarding and these tables can be modified byhardware and/or software. However, because such tables may typically beintegrated on a single chip to allow for line rate lookups, there arepractical limitations as to the possible sizes of the tables. Theselimitations include practical die size factors that limit the number oflookup table entries that can be stored on-chip.

One possible solution is to locate the lookup tables externally (i.e.off-chip), but this arrangement may still suffer from limited sizeand/or lack of flexibility in making the most efficient use of theavailable memory. Another possible solution is to extend each of theinternal tables to also use external memory. Such a conventional systemstructure is shown in FIG. 1 and indicated by the general referencecharacter 100. In FIG. 1, Router/Switch 102 includes Forwarding Engine104, External IP Table Interface 106, and External MAC Table Interface108. The IP table can be extended to include IP Table External Memory110. Similarly, the MAC table can be extended to include MAC TableExternal Memory 112. However, drawbacks of this conventional approachinclude increased system costs due to likely back-end board areaincreases, primarily due to more pins needed on the package to supportmultiple external memory interfaces. Further, such conventionalapproaches typically do not support flexible allocation of the externalmemory. Accordingly, chips may have inadequate or limited IP tableand/or MAC table entries for many applications. Further, this approachmay be limited by physical memory and associated interface limitations,such as the refresh rate requirement of dynamic (e.g., DRAM). Overall,negatives such as an increased number of pins on the package due tosupporting multiple interfaces drives up the cost of the system and,along with a lack of flexible allocation, makes this approachundesirable.

Consequently, what is needed is a solution that can flexibly allocateexternal memory and can effectively extend internal lookup tableswithout using multiple, relatively costly, memory interfaces.

SUMMARY

The invention overcomes the identified limitations and provides asolution that can flexibly allocate external memory and can effectivelyextend internal lookup tables without using multiple, relatively costly,memory interfaces.

According to embodiments, a configurable lookup table extension systemincludes a plurality of lookup tables arranged in an internal memory, anexternal memory, and a flexible controller configured to couple at leastone of the plurality of lookup tables to the external memory through asingle memory interface. Implementations of this system can support theflexible allocation of IP and MAC table entries so that a router/switchcan flexibly support applications suited to a particular allocation.

According to another aspect of the embodiments, a method of extending alookup table includes arranging a plurality of lookup tables in aninternal memory, providing an external memory, and flexibly controllinga coupling of at least one of the plurality of lookup tables to theexternal memory.

This approach provides an efficient scheme for extending multipleinternal tables to external memory via a single external memoryinterface. Further, such extensibility is also programable to allow thesize and number of external tables to be configured by software. Thissolution can provide the flexibility of customizing table sizes fordifferent markets and/or customer requirements.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention are described with reference to the FIGS,in which:

FIG. 1 depicts a conventional lookup table extension system.

FIG. 2A depicts an exemplary interface structure between a router/switchand an external memory according to an embodiment.

FIG. 2B depicts a relation between internal lookup tables and anexternal memory according to an embodiment.

FIG. 3 is a block diagram depicting a table lookup request according toan embodiment.

FIG. 4 is a detailed block diagram of an example implementation of atable lookup extension system according to an embodiment.

FIG. 5 is a timing diagram showing packet accesses as related toexternal memory banks for one channel of the example implementation ofFIG. 4.

FIG. 6 depicts flexible external memory allocation according to anembodiment.

DETAILED DESCRIPTION

Embodiments of the invention are described with reference to specificdiagrams depicting system arrangements and methods. Those skilled in theart will recognize that the description is for illustration and toprovide the best mode of practicing the invention. The description isnot meant to be limiting. For example, reference is made to severaltypes of forwarding lookup tables, including IP and MAC types, but theinvention is applicable to other types of lookup tables as well.Further, types of memory implementation, including numbers of memorybanks and the like, merely provide example implementations and shouldnot be construed as limiting.

Referring now to FIG. 2A, a depiction of an exemplary interfacestructure between a router/switch and an external memory according to anembodiment is indicated by the general reference character 200.Router/Switch 202 can interface through a single Interface 206 toExternal Memory 204. The single interface can include, for example, Ndata or DQ signals. Also, the same interface can include M Addresssignals. In one aspect of the invention N is 36 and M is 20. Further,the single interface can include a plurality of memory bank signals ifthe external memory is so organized.

Referring now to FIG. 2B, a relation between internal lookup tables andan external memory according to an embodiment is shown and indicated bythe general reference character 250. Router/Switch 252 can includeForwarding Engine 254, Internal Tables 256, and External MemoryInterface 258. Internal Tables 256 can include a plurality of tables,such as IP Table 256A, MAC Table 256B, and Forwarding (FRW) Table 256N.Interface 260 can connect External Memory 262 to Router/Switch 252. Thisapproach can also dynamically share the external memory between, forexample, IP and MAC tables. So, if a certain application required moreIP lookup table space and less MAC lookup table space, this could be soallocated for more efficient memory usage.

Referring now to FIG. 3, a block diagram depicting a table lookuprequest according to an embodiment is shown and indicated by the generalreference character 300. Table Lookup Request 302 can pass throughSwitch 304 for either an internal memory access or an external memoryaccess. If the access is designated to be an external access, therequest can pass to Flexible Interface 306. If the access is designatedto be an internal access, the request can be passed to Local Memory 308.For the external access, Flexible Interface 306 can couple to ExternalMemory 310. There are a number of different ways to choose whether thereneeds to be an external memory access, such as by using a memory map ora binary search operation where part of a tree is internal and part isexternal. According to embodiments, software can designate whichaccesses can be routed to external memory. Further, software can manageexternal interface accesses based on the lookup table type.

Referring now to FIG. 4, a detailed block diagram of an exampleimplementation of a table lookup extension system according to anembodiment is shown and indicated by the general reference character400. This very particular example case shows two channels, each passingthrough a single memory interface. Of course, embodiments may have onlyone channel or any number of channels. Tables 402 can interface toeither channel via shared bus 442. Tables 402 can include any number ofdifferent lookup table types. The examples shown in FIG. 4 includeLongest Prefix Match (LPM) 404A, MultiCast (MC) 404B, Next Hop Table(NHT) 404C, Forwarding (FRW) 404D, Media Access Control Source Address(404E), and Media Access Control Destination Address (404F). Asindicated, the tables shown herein are merely examples of types oftables that may be used in embodiments. Of course, other types of tablesand/or combinations of such tables may also be used in an embodiment.

The system embodiment design considerations can include the speed andsize of the internal and external memories, the system clockingfrequency, the table types, and the associated number of accesses perpacket that are required. The flexible scheme allows software toallocate different accesses to different tables. Further, in order tomeet line rate operation, a number of table lookups need to be performedin a fixed amount of time. This can limit the number of accesses toexternal memory that are effectively available. Also, depending on thetype of external memory used, there may be particular inefficienciesintroduced by physical properties of the memory, such as refresh and/orTrc (row or bank cycle time) for a DRAM type of memory. Another possiblelimitation is the speed of operation of the external memory. The fasterthe memory, the more accesses that can be made available per packet.These considerations can determine the number of accesses available perpacket on the shared memory interface. A given table, for example, mayneed two or more lookups or accesses per packet while another type oftable may only require one lookup per packet. Considering the number oftables to be looked up and the number of accesses per table, the totalnumber of accesses available may be determined. The flexible externalinterface according to this embodiment can allow mapping of theavailable or possible accesses to the required accesses by lettingsoftware configure a mapping of a particular table lookup to aparticular external memory access. Associated software can alsodesignate which tables are to go externally for their accesses. In thisway, the external memory can essentially function as an extension of theinternal memory.

In FIG. 4, the first channel can include Flexible Allocation 406 andAccess Queues 408. The flexible allocation can allow software to map anyof the tables to different available accesses, as will be shown in moredetail below. This flexible allocation can be implemented in any numberof ways, including as a series of suitably controlled multiplexersand/or switches. Access Queues 408 can include, for example, LPM1 410A,LPM2 410B, MC/LPM3 410C, FRW 410D, BMP 410E, and NHT 410F as tablesdesignated for external memory access. Other example implementationsaccording to embodiments can include more or less than the six accessqueues, as shown in FIG. 4. Note that “MC/LPM3” can indicate an accessdesignated for an MC type table or an LPM type table. Multiplexer 412can allow a selected table access to pass through to the external memoryin response to signals from Control 414. The selected access can pass toMemory Control 416, then to I/O Control 418, and finally to ExternalMemory 420. The response, or result, of a table lookup from the extendedtable in external memory for the first channel can be returned viaResponseQCh1 432 to Flexible External Controller 436. The flexibleexternal controller can send responses back via signals 444 to completethe lookup table extension for Tables 438. Tables 438 can include, forexample, the same lookup table types as in Tables 402. Accordingly,Tables 438 can include LPM 440A, MC 440B, NHT 440C, FRW 440D, MACSA440E, and MACDA 440F.

Similarly, the second channel in this very particular example caninclude Flexible Allocation 422 and Access Queues 424. Access Queues 424can include, for example, LPM1 426A, LPM2 426B, MC/LPM3 426C, an unusedslot or extra access 426D, MACDA 426E, and MACSA 426F as tables selectedfor external memory access. Other example implementations according toembodiments can include more or less than the six access queues, asshown in FIG. 4. Multiplexer 428 can allow a selected table access topass through to the external memory in response to signals from Control430. The selected access can pass to Memory Control 416, then to I/OControl 418, and finally to External Memory 420. The response, orresult, of a table lookup from the extended table in external memory forthe second channel can be returned via ResponseQCh2 434 to FlexibleExternal Controller 436. Further, the flexible external controller cansend responses back via signals 444 to complete the lookup tableextension for Tables 438, as discussed above.

Referring now to FIG. 5, a timing diagram showing packet accesses asrelated to external memory banks for one channel of the exampleimplementation of FIG. 4 is shown and indicated by the general referencecharacter 500. In this example, the external memory can be organized as8 separate banks that can all be included in the single memoryinterface, according to an embodiment. In FIG. 5, the access activitywaveforms for each of the eight banks are labeled “Bank 1” through “Bank8.” Also, there are six cycles allocated to initiate lookups in apredominately round-robin fashion, although any suitable ordering couldbe used. Note that certain table types may have their lookups brokeninto several portions, such as “LMP1” and “LMP2.” Software can be usedto determine, for example, the ordering of the requests, such as the MCrequest in the third position. This is adaptable to how many accessesare available and how many accesses are needed. The selection accordingto embodiments can be done based on a particular configuration of thechip.

In FIG. 5, the Clock cycle time can be about 2.5 ns, so the clockfrequency can be about 400 MHz. Each of the access/allocation cyclesshown can coincide with the cycle time of the external memory, which isabout 20 ns or 8 Clock cycle times in this example. In this example, an“access” may be completed in one Clock cycle while an “allocation” of aparticular memory bank may last 8 Clock cycles. A first access cycle,which can be an LPM1 table access can be initiated for Bank 1 at Clockcycle. A second access cycle, which can be an LPM2 table access can beinitiated for Bank 2 at Clock cycle. A third access cycle, which can bean MC table access can be initiated for Bank 3 at Clock cycle 3. Suchstaggered access/allocation patterns can continue for each bank throughBank 8, as shown. Once Bank 1 is ready for another access after itscycle time (Trc) has expired, it can be re-allocated and another accesscan begin at Clock cycle 9. The pattern of accesses/allocations as shownin FIG. 5 is merely exemplary and any suitable pattern to relate thenumber of table access available in an access queue to the number ofexternal memory banks constituting a single memory interface can beimplemented according to embodiments. Also, the number of Clock cyclesper “access” or “allocation” is merely exemplary in FIG. 5 and such arelation may vary depending on clock speed and/or system requirements inother implementations, according to embodiments.

Referring now to FIG. 6, a flexible external memory allocation accordingto an embodiment is shown and indicated by the general referencecharacter 600. The entry width as well as the depth (i.e., the number oftable entries) can be adjusted by the flexible interface to allowdifferent table sizes. As shown, the example is for different tableadjustments for two channels: a First Channel and a Second Channel,which can correspond to those discussed above with reference to FIG. 4.In the example of FIG. 6, Table 1 (602), Table 2 (604), and Table 3(606) may have the same width, but different depths. These tables mayspan across the space allowed for both channels. Further, Table 4 (608),Table 5 (610), and Table 6 (612) can have different width and depthcombinations, with Table 4 used for the first channel and Tables 5 and 6used for the second channel, as only one example case. Accordingly, theexternal memory can be partioned into different portions of differentsizes. These portions can then be allocated to associated tables toallow the effective size of each table to be customized to meet userand/or market requirements.

Advantages of embodiments of the invention include the extensibility oflookup tables, the ability to multiplex multiple tables through a singlememory interface, and the configurability of the effective table sizes.

Having disclosed exemplary embodiments and the best mode, modificationsand variations may be made to the disclosed embodiments while remainingwithin the subject and spirit of the invention as defined by thefollowing claims.

1. A configurable lookup table extension system, comprising: at leastone internal memory having a plurality of lookup tables; at least oneexternal memory; and a flexible controller configured to couple the atleast one internal memory with the plurality of lookup tables with theat least one external memory through a single memory interface, whereinthe flexible controller dynamically allocates the at least one externalmemory in accordance with the plurality of lookup tables therebyallowing for an efficient usage of the at least one external memory. 2.The configurable lookup table extension system of claim 1, wherein: theat least one internal memory includes static random access memory(SRAM); and the at least one external memory includes dynamic randomaccess memory (DRAM).
 3. The configurable lookup table extension systemof claim 1, wherein: the at least one internal memory and the at leastone external memory include static random access memory (SRAM).
 4. Theconfigurable lookup table extension system of claim 1, wherein: theplurality of lookup tables includes Internet Protocol (IP) and MediaAccess Control (MAC) type tables.
 5. The configurable lookup tableextension system of claim 4, wherein: the plurality of lookup tablesfurther includes IP MultiCast (MC), IP Next Hop Table (NHT), and IPLongest Prefix Match (LPM) type tables.
 6. The configurable lookup tableextension system of claim 1, wherein: at least one of the plurality oflookup tables is configured for packet processing; the at least oneexternal memory is arranged as a plurality of banks; and the flexiblecontroller includes an access queue configured to couple a packet accessto at least one of the plurality of banks.
 7. The configurable lookuptable extension system of claim 1, wherein: the single memory interfaceincludes a plurality of data and address signals.
 8. The configurablelookup table extension system or claim 1, wherein: the at least oneexternal memory includes a plurality of table allocations, each tablehaving a configurable depth span and a configurable width span.
 9. Theconfigurable lookup table extension system of claim 8, wherein: theplurality of table allocations includes at least one table having theconfigurable width span of at least two channels.
 10. A method ofextending a look up table, comprising: providing at least one internalmemory with a plurality of lookup tables; providing at least oneexternal memory; and providing a flexible controller configured tocouple the at least one internal memory with the plurality of lookuptables to the at least one external memory through a single memoryinterface, wherein the flexible controller dynamically allocating the atleast one external memory between the plurality of lookup, tablesthereby allowing for an efficient usage of the at least one externalmemory.
 11. The method of extending the lookup table of claim 10,wherein: the at least one internal memory includes static random memory(SRAM); and the at least one external memory includes dynamic randomaccess memory (DRAM).
 12. The method of extending the lookup table ofclaim 10, wherein: the at least one internal memory and the at least oneexternal memory include static random access memory (SRAM).
 13. Themethod of extending the lookup table of claim 10, wherein: the pluralityof lookup tables includes Internet Protocol (IP) and Media AccessControl (MAC) type tables.
 14. The method of extending the lookup tableof claim 13, wherein: the plurality of lookup tables further includes IPMultiCast (MC), IPNext Hop Table (NHT), and IP Longest Prefix Match(LPM) type tables.
 15. The method of extending the lookup table of claim10, wherein: at least one of the plurality of lookup tables isconfigured for packet processing; the at least one external memory isarranged as a plurality of banks; and the flexible controller includesan access queue configured to couple a packet access to at least one ofthe plurality of banks.
 16. The method of extending the lookup table ofclaim 10, wherein: the single memory interface includes a plurality ofdata and address signals.
 17. The method of extending the lookup tableof claim 10, wherein: the at least one external memory includes aplurality of table allocations, each table having a configurable depthspan and a configurable width span.
 18. The method of extending thelookup table of claim 17, wherein: the plurality of table allocationsincludes at least one table having the configurable width span, theconfigurable width span including at least two channels.